<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script type="text/javascript">
        function onchangeFun() {
            alert("已改变");
        }

        window.onload = function () {
            var change = document.getElementById("change01");
            change.onchange = function () {
                alert("改变了");
            }
        }
    </script>
</head>
<body>
    请选择
    <select onchange="onchangeFun()" >
        <option>11</option>
        <option>22</option>
        <option>33</option>
        <option>44</option>
    </select>
        <select id="change01" >
        <option>21</option>
        <option>32</option>
        <option>43</option>
        <option>54</option>
    </select>
</body>
</html>